Mild steel and stainless steel are mainly used as metallic materials for engine cylinder head gasket materials for which water resistance, LLC (long life coolant) resistance, and heat resistance are required; however, when a vulcanizing adhesive is directly applied to such a metallic material to bond the material to rubber, liquid resistant adhesion durability is poor. When this rubber-metal laminated plate is subjected to an dipping test in which the plate is dipped in water, LLC, etc., adhesive peeling occurs.
The adhesion between a metal plate and rubber is improved by using, for example, zinc phosphate treatment, iron phosphate treatment, or other methods for treating the surface of a stainless steel plate. In these methods, a degreased steel plate is dipped in an acidic chemical solution to form an insoluble film on the steel plate, thereby ensuring rust prevention and adhesion to an adhesive. However, these methods had the following problems: sludge (industrial waste) was generated at the stage of forming a film; it was necessary to always supply chemical agents because chemical components were reduced due to the reaction; and the cost was high.
The present applicant has previously proposed improving resistance to water, LLC, etc., by performing coating type chromate treatment on stainless steel as a pretreatment before a vulcanizing adhesive is applied (Patent Documents 1 and 2). However, the coating type chromate treatment, which includes Cr6+ ions, is not preferable in terms of environmental protection.
Moreover, as a method that does not use zinc phosphate, iron phosphate, chromium, etc., Patent Document 3 has proposed a surface treating agent containing an organic resin, silica, and titanium and zirconia derived from fluoride. However, since this surface treating agent contains fluorine, defluoridation of wastewater is necessary, thus requiring complex facilities.
In contrast, Patent Document 4 has proposed a fluorine-free treating agent as a modified version of the above surface treating agent. However, sufficient adhesion cannot be obtained in severe use environments such as for cylinder head gasket etc., which is exposed to high temperature LLC vapor.